A volcanic mountain with a wide base and gently sloping sides is called a shield volcano.

Here's why:

* Wide Base: Shield volcanoes are formed by highly fluid lava flows that spread out over a large area.

* Gently Sloping Sides: The lava flows, being thin and runny, cool slowly and build up the volcano with gradual slopes.

Some famous examples of shield volcanoes include:

* Mauna Loa in Hawaii

* Kilauea in Hawaii

* Mount Etna in Sicily